feat: add fixed_axis keyword to enable 2d pressure coupling

[ape33]

This PR solves issue #525

[pq-perf-bot]

⚡ Performance (instruction count) — ✅ no regressions

per-benchmark breakdown

benchmark	base Ir	PR Ir	Δ
bondedForces	38.24M	38.24M	+0.00%	✅
boxTransforms	10.67M	10.67M	+0.00%	✅
constraints	11.06M	11.06M	+0.00%	✅
coulombKernel	5.80M	5.80M	+0.00%	✅
forceKernel	14.16M	14.16M	-0.00%	✅
integrator	32.91M	32.91M	-0.00%	✅
interWater	1681.41M	1681.41M	+0.00%	✅
kinetics	8.55M	8.55M	+0.00%	✅
linearAlgebra	2.08M	2.08M	+0.00%	✅
nonCoulombPairs	5.48M	5.48M	+0.00%	✅
shiftVector	5.81M	5.81M	+0.00%	✅
virial	9.10M	9.10M	-0.00%	✅

_{Deterministic callgrind instruction counts vs the base branch; gated at ±2%. Not wall-clock.}

[codecov]

Codecov Report

❌ Patch coverage is 33.81295% with 92 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.78%. Comparing base (ba3bc3d) to head (ffa85e7).

Files with missing lines	Patch %	Lines
src/manostat/stochasticRescalingManostat.cpp	17.07%	34 Missing ⚠️
src/manostat/berendsenManostat.cpp	21.21%	26 Missing ⚠️
src/input/inputFileParser/manostatInputParser.cpp	15.78%	16 Missing ⚠️
src/settings/manostatSettings.cpp	6.66%	14 Missing ⚠️
src/setup/manostatSetup.cpp	93.54%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #526      +/-   ##
==========================================
- Coverage   91.31%   90.78%   -0.53%     
==========================================
  Files         319      319              
  Lines       14717    14826     +109     
  Branches     4909     4948      +39     
==========================================
+ Hits        13439    13460      +21     
- Misses       1192     1280      +88     
  Partials       86       86              

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